Microphone aperture

ABSTRACT

Microphone array for achieving a substantially frequency-independent directivity using a plurality of microphones disposed along a rectilinear array. The rectilinear array is at least as long as the wavelength of the lowest frequency, where a useful directivity is desired. The rectilinear array has a first end and a second end. The microphones close to the first end are intended for the highest frequencies and the microphones close to the second end are intended for the lowest frequencies. The mutual spacing of the microphones is frequency-dependent. The signals from the individual microphones are band-pass filtered, the passbands and cut-off frequencies of the individual band-pass filters being adapted to the frequency band the individual microphones are intended for. The individual band-pass filters are adapted such that the amplitude of the summated signal after band-pass filtering is substantially the same when a sinus-shaped test signal is used, the amplitude of said test signal being constant and the frequency of said test signal varying within the frequency range where the microphone array is to have a substantially frequency-independent directivity.

TECHNICAL FIELD

The invention relates to a microphone array for achieving asubstantially frequency-independent directivity using a plurality ofmicrophones disposed along a rectilinear array.

BACKGROUND ART

A microphone array of this type can for example be used for recordings,where a frequency-independent directivity is desirable. Microphones areinter alia characterised by their sensitivity to different frequencies,but also by their sensitivity to the angle of incidence of the soundwaves into the microphone. A microphone may, for example, have aspherical characteristic, where it receives sound waves substantiallyequally well from all angles, however, a microphone may also have a moreor less conical directional characteristic. Thus, the microphone ishighly sensitive to sound waves coming from a particular direction andless sensitive to sound waves coming from other directions. Whenmicrophones are used for the recording or transmission of, for example,music in a recording studio or a concert hall, the selection of thetypes of microphones used depends on a number of circumstances, such as,for example, the instrumentation in question, the acoustic environmentin the recording room and the desired acoustic pattern. In order to beable to create an optimum recording under a multitude of differentconditions, it is required that a large number of different types ofmicrophones is available. Usually, many microphones are used for a taskat hand, said microphones being moved around and exchanged with respectto the requirements that may arise. For example, a microphone may berequired, where the directivity of said microphone may be improved withrespect to existing types, while altering the frequency dependence ofthe directivity, the basis thereof being a microphone with constantfrequency and an improved directivity in a larger frequency range. Thus,the same microphone may be adapted electronically to different needs.The number of different types as well as switching between said typesmay thus be achieved in a considerably easier and more flexible way.

Further advantages become apparent, if the same microphone could be madeto focus on several adjustable directions simultaneously, thuspossessing individually adjustable directivity characteristics for eachof these directions. Depending on the actual acoustic conditions, such amicrophone may replace a varying number of conventional typemicrophones, at the same time achieving improved results and less timeconsumption in the recording room.

Thus, there is a need for a microphone with controllable, substantiallyfrequency-independent directivity, i.e. the directivity within aconsiderable frequency range is substantially the same, or saidmicrophone possessing a preselected characteristic, said characteristicbeing improved with respect to conventional microphones. It isadvantageous that the system is designed in such a way that it is ableto focus on several points in space simultaneously.

Systems fulfilling these needs to a varying degree are well-known in theart. U.S. Pat. No. 5,657,393 discloses an elongated microphone arraywith a plurality of microphones disposed in groups depending on theirfrequencies, said groups either being disposed adjacent each other oralong an elongated array. The system makes use of band-pass filters foreach group of microphones, and the resulting signals behind theband-pass filter are summated, and the resulting signal is a signal withhigh directivity. The system shows a good directivity characteristic inthe direction of the elongated array, however, a system of this type isdisadvantageous in several ways. The two major disadvantages areinstabilities arising at the transition from one group to the next andthus instabilities arising in the frequency-dependent directivitycharacteristic because of the grouping of microphones according tofrequencies. Since the elongated array has a physical extension so thatthe sound signals reach the individual microphones at different times, atime correlation is used to establish the desired directivitycharacteristic. A microphone array of this type is often referred to asan “end-fire” microphone.

Joseph Lardies: “Acoustic ring array with constant beamwidth over a verywide frequency range”, Acoustic letters, Vol. 13, no. 5, p. 77-81discloses a technique for maintaining the beamwidth of a transducerconstant over a frequency range of N octaves. An acoustical ring arrayof six sensors is used to produce a radiation pattern at a givenfrequency, whereas a half-scale model is implemented to give the samedirectivity pattern at the double frequency. Compensation filters areused in the respective array outputs to produce a constant beamwidthover the corresponding octave. The design process can be repeated Ntimes in order to obtain an acoustical array with constant beam widthover a frequency range of N octaves. However, the beam width is onlyconstant in the plane of the acoustical array. Furthermore, thetechnique uses eighth-order Butterworth band-pass filters, which havevery sharp cut-off frequencies and a flat response in the passband. As aresult, the transducer has very distinct sidelopes, which means thedirectivity of the transducer is very poor. The article does not mentionor suggest any means to change the directivity of the transducer.

WO 0158209 discloses a system having a number of microphones disposed ina circle for recording a sound field: The document provides a thoroughanalysis of the frequency characteristic for such a system and it isshown, how the amplification in the system depends on the number ofmicrophones, and which frequencies are observed. The disclosed examplesshow a strong frequency dependence with respect to amplitudeinformation, and the system for processing the signals is relativelycomplicated.

WO 0171687 discloses a surveillance system, where a network ofmicrophones is used to monitor conversations in a large room. A specialdevice is equipped with a large number of microphones in order to obtainhigh directivity, but this only succeeds at the cost of the frequencyinformation.

U.S. Pat. No. 6,317,501 discloses a system having a network ofmicrophones, said network being used to obtain directional informationfrom incident sound. The system uses filters and time delays to generatean output signal. The system is specifically designed to finddirectional information in a sound field.

U.S. Pat. No. 6,526,147 describes an elongated microphone array withpairs of microphones disposed on each side of the microphone array. Themicrophones are arranged equidistantly. The signal from each pair ofmicrophones is summated and transmitted to a filter, and the resultingfiltered signals are summated. However, the results shown display acertain frequency-dependent directivity.

U.S. Pat. No. 4,696,043 shows a microphone array with microphonesdisposed equidistantly, a network with weighting factors being used toalter the directivity characteristic of the system. It is shown that agreat number of different directivity characteristics are obtained,however, said characteristics are highly frequency-dependent.

U.S. Pat. No. 5,058,170 discloses a directional microphone arrayprovided to suppress acoustic feedback and howling generated byloudspeaker systems.

U.S. Pat. No. 5,473,701 discloses a system for use with mobiletelephones, where two microphones are used to obtain high directivity.This is achieved by means of inter alia delay circuits and low-passfilters.

US Patent Application No. 20020069054 discloses a system having a numberof microphones, said microphones apparently rotating in space by meansof time delays. The document also states that the system can focus onseveral points simultaneously.

DISCLOSURE OF INVENTION

Therefore, there is a need for a microphone or a microphone arraypossessing a directivity, which has controllable characteristics and issubstantially frequency-independent, and where the degree of frequencydependence is selected. This is achieved by means of a microphone arrayaccording to the characterising portion of claim 1.

When the individual microphones of the microphone array are disposeddepending on their frequency, and the band-pass filters are adjusted tothe individual microphones with respect to their location in the array,frequency characteristic of the directivity is improved considerably,especially for low frequencies, but also for high frequencies.

Finally, the individual signals from the individual microphones may eachbe recorded separately, and the desired directivity may be determined ata later stage by means of band-pass filtering and summation.

The invention also relates to a microphone arrangement comprising atleast two of the above-mentioned microphone arrays, where the at leasttwo microphone arrays are arranged in one plane.

In this connection, it is, for example, conceivable to dispose two ofthe above-mentioned microphone arrays along one axis, whereby someparticularly beneficial properties are obtained with respect to thedirectivity of the microphone arrangement.

In another embodiment of the invention the microphone arrays aredisposed along radii extending from the centre of an imagined circle,the first ends of the microphone arrays facing the centre. Themicrophone arrays are preferably disposed in such a way that at leasttwo different individual microphones from different microphone arraysare disposed along imagined concentric circles having the same centre.

Thus, an even better directivity is obtained.

In a preferred embodiment of the invention, the shortest circular arcspacing between microphones on the circle closest to the centresubstantially corresponds to or is smaller than the radial distancebetween the two circles closest to the centre. In a particularlypreferred embodiment of the invention, the signals from the individualmicrophones are each independently associated with time delays selectedin such a way that the effect of the microphone arrangement is focusedin at least one direction and/or against one punctiform area in front ofthe microphone apparatus.

Thus, several directivities and/or focusing areas may be obtainedsimultaneously by selecting the correct time delays, said directivitiesand/or focusing areas having the same efficiency.

In a further embodiment of the invention, the individual band-passfilterings for summated signals from the individual microphones on thesame circular arc are carried out after the signals from the microphoneshave been time-delayed.

In a particularly preferred embodiment the signals from the individualmicrophones are run through several sets of time delays and/or severalsets of band-pass filters.

Thus, even more directivities may be obtained without negative impact onthe efficiency.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained below by way of embodiments and withreference to the drawings, in which

FIG. 1 shows a microphone array with microphones disposed along arectilinear array, also known as “end fire”,

FIG. 2 shows a microphone array according to the invention withposition-dependent time delay and position-dependent band-passfiltering,

FIG. 3 shows the sensitivity of the microphone array with respect to twofrequencies spaced mutually far apart,

FIG. 4 shows the structure of band-pass filters for correspondingindividual microphones in the figures,

FIG. 5 shows the sensitivity of the microphone array with respect tothree frequencies spaced close to each other,

FIG. 6 shows a microphone array having 32 microphones as well as thesensitivity of said microphones as a function of the frequency,

FIG. 7 shows the directivity of a microphone array without band-passfilters,

FIG. 8 shows a microphone array according to the invention withband-pass filters,

FIG. 9 shows an alternative embodiment of the microphone array accordingto the invention.

FIG. 10 shows a microphone arrangement according to the inventioncomprising a number of microphone arrays according to the invention, and

FIG. 11 shows a further microphone arrangement according to theinvention.

BEST MODE(S) FOR CARRYYING OUT THE INVENTION

The invention is explained below by way of an example, but it will beunderstood that the invention is not limited to this example.

FIG. 1 shows a microphone array 1 having a reference end 2 and a soundsource 3 as well as a direction towards the sound source 4. An array ofthis type is often referred to as an “end-fire” microphone. Themicrophone array shown is a rectilinear element with individualmicrophones 5 disposed along the longitudinal axis, said microphonebeing disposed with the smallest spacing in the direction towards thesound source and a wider spacing away from the sound source. Basically,the length of the microphone array is at least as long as the wavelengthof the lowest frequency, for which a high directivity is desired. Thelowest frequency must be selected with care, as very low frequenciesresult in long microphone arrays of up to several meters in length.Moreover, at very low frequencies it is also doubtful, how much isachieved by a high directivity, since the human ear does not well pickup directional information on deep sounds.

The positioning of the individual microphones in the microphone array isfrequency-dependent, the position of said individual microphones beingfound using the following expression:

$l_{n} = {l \cdot 2^{- {(\frac{N - n}{d})}}}$wherein 1 is the length of, for example, the longest wavelength, forwhich frequency independence is desired, 1.sub.n is the position of then'th microphone, N is the maximum number of microphones and d is thenumber of microphones per octave. Thus, the ratio between N and ddetermines the number of octaves to be covered by the microphone arrayor, in other words, the frequency range of the microphone array.However, the above-mentioned formula for the positions of the individualmicrophones is not the only way to describe the positioning of saidmicrophones. Overall, the important factor is that the centre-to-centredistances between the individual microphones are the same, whenfrequency is taken into consideration. Below there is an example of twofrequencies f₁ and f₂, where the frequency for f₂ is twice as high asfor f₁ and where five microphones are considered:

f₂ = 2 ⋅ f₁${f_{1}\text{:}\mspace{14mu} m_{1}} = \frac{{\sum\limits_{n_{1}}^{n_{1} + 5 - 1}l_{n + 1}} - l_{n}}{5 - 1}$${f_{2}\text{:}\mspace{14mu} m_{2}} = \frac{{\sum\limits_{m_{1}}^{m_{1} + 5 - 1}l_{m + 1}} - l_{m}}{5 - 1}$m₁ ≅ 2 ⋅ m₂,wherein m₁ is a first mutual spacing between two microphones providedfor a first frequency f₁, and m₂ is a second mutual spacing between twomicrophones provided for a second frequency f₂.

When the microphones are arranged in this manner, their positions becomefrequency-dependent.

Since the individual microphones are disposed along the microphonearray, the sound from the sound source 3 reaches the individualmicrophones at different times. The individual time delays of thesignals from said individual microphones are used to establishdirectivity. The time delays are calculated based on the propagationvelocities and the differential distances between the microphones basedon the direction, where the maximum sensitivity of the array is desiredto be. If the sound source is placed along the axis 4 of the microphonearray, the sound arrives at the individual microphones with a timedifference, but since the signals are time-delayed, they appear to reachthe individual microphones at the same time. Thus, a high directivity ofthe signal is obtained, since the signals from the individualmicrophones upon summation amplify each other, while the sound wavescoming from other sound sources not placed along the axis 4 of themicrophone array 1 reach the individual microphones at other times andwill thus be strongly attenuated. Relatively speaking, however,depending on the direction, some particular angles of incidence continueto amplify signals more than other angles of incidence. This phenomenonis known as grating loops or sidebands, and is well-known to a personskilled in the art, therefore it will not be explained further.

The basic idea of the microphone array according to the invention is toachieve a substantially frequency-independent directivity. In practice,completely frequency-independent directivity is, of course, impossible,however, it is possible to provide said directivity with a high degreeof frequency independence. This is only achievable, when certainparticular conditions are met, viz. that the individual microphones aredisposed in a frequency-dependent pattern along the microphone array, asdescribed above. Subsequently, the individual signals from theindividual microphones are time-delayed depending on their position,resulting in a summated signal, where sound waves incident with the axis4 of the microphone array 1 are summated constructively, whereas soundwaves incident at an angle to the microphone array are summateddestructively to a greater or lesser degree. After theposition-dependent time delay Z, the signals are run through band-passfilters F, the passbands and cut-off frequencies of said filters beingdependent on the frequency band for which the individual microphones areintended. After band-pass filtering, the individual signals are summatedS, and the resulting output signal O has substantiallyfrequency-independent directivity, if both the frequency-dependentpositioning of the microphones, the position-dependent time delay andband-pass filtering have been chosen correctly. It should be noted thatthe directivity of the microphone array may be altered, depending onwhat is desired, from having a very high directivity to having a verylow directivity, if the pass bands of band-pass filters F are altered.This is carried out solely by altering the band-pass filters, i.e.without making any physical alterations to the microphone array. Ifnecessary, the same signals may be run through several differentband-pass filters and the same microphone array may therefore possessseveral different directivity characteristics at the same time. This isespecially important in connection with a two-dimensional orthree-dimensional array, as described below.

FIG. 3 shows a microphone array according to the invention, where theband-pass filters of the individual microphones are disposed accordingto frequency, depending on the frequency band the individual microphonesare intended for. This means that the microphones to the left closest tothe sound source and being disposed with the smallest spacing isprovided for the highest frequencies, wherefore the passband of theband-pass filter is set for a high frequency. The microphones furtheraway from the sound source are intended for lower frequencies, whereforethe passbands of the band-pass filters are set for lower frequencies.Upon applying a signal with two discrete frequencies F1 and F2 beingspaced far apart in the frequency band, the following occurs. Allmicrophones receive both frequencies, but the sensitivities of theindividual microphones with respect to the individual frequencies F1, F2are different because of band-pass filtering. Thus, the sensitivity tothe high frequency F1 is highest at the microphones close to the soundsource, since the band-pass filters are set for high frequencies,whereas the sensitivity to the low frequency F2 is highest at themicrophones away from the sound source. Therefore, the sensitivitiesshown are not the sensitivities of the microphone array, but thesensitivity of the individual microphones to the two frequenciesdepending on the positions of the microphones. This sensitivitydescribes the term aperture of a microphone array. Thus, an aperture(“opening”) provides selectivity for the individual microphones withrespect to the individual frequency bands, thus making it possible tocontrol the interdependence of frequency band and microphone position toa great extent, which is an important aspect in this connection.

FIG. 4 is a schematic representation of the passbands and cut-offfrequencies of the band-pass filters for a microphone array having 11microphones M1-M11. The illustrated microphone array may either beconceived as a greatly simplified embodiment or a section of a largerarray. As illustrated, the band-pass filter associated with themicrophone for the highest frequencies, i.e. microphone M1, ispositioned at a high frequency and cuts off the lowest frequencies,whereas the microphone intended for the lowest frequencies, i.e.microphone M11, has a band-pass filter cutting off the upperfrequencies. In FIG. 4, the band-pass filters of the individualmicrophones are highly representational, the most important informationbeing that the passbands and cut-off frequencies are different for theindividual microphones. In order to achieve frequency-independentdirectivity, the centre frequency of the band-pass filters must have thesame exponential curve as the mutual spacing between the microphones. Atthe same time, the ratio in percent between the bandwidth of theband-pass filters and the centre frequency must be constant. This isalso referred to as constant relative bandwidth. If it is desired tochange the directivity while maintaining its frequency independence, allband-pass filters must be altered simultaneously and with the samepercentage. However, if it is desired to vary the frequency response ofthe directivity, this is achieved by altering either the centrefrequency or the bandwidth of the filters.

In FIG. 5, four frequencies F1-F4 are plotted, said frequencies beingspaced comparatively closely in the frequency band. If these fourfrequencies are applied to the microphone array of FIG. 4, thesensitivity shown in FIG. 5 to said four frequencies is obtained. As isapparent, the sensitivities of the microphones of the microphone arraywith respect to the individual frequencies are different, and therefore,the resulting signals from the individual microphones depend on whichfrequency is observed. The sensitivities to the four frequenciesdecrease or are reduced when reaching the outer limits of the activeapertures at F1-F4. This reduction in the microphone sensitivity mayalso be referred to as a weighting of the signals or, depending on thepoint of view, an apodisation of the active aperture for frequenciesF1-F4.

Another important factor is the selection of band-pass filtering for theindividual microphones. Although the passbands of the individualband-pass filters are positioned at different frequencies, a signal of agiven frequency generates a signal from all band-pass filters, saidsignal being attenuated to a greater or lesser degree. For correctsummation of the signals it is important that the signals are in phase,regardless of the attenuation from a given filter. This can only beachieved using digital filters with a pole position, resulting in aconstant group propagation time within the entire frequency range used.

Below is an example illustrating the resulting amplification with eightmicrophones for two frequencies f₁ and f₂ having the same amplitude.

M1 M2 M3 M4 M5 M6 M7 M8 SUM f₁ 0.05 0.1 0.7 1.0 1.0 0.7 0.1 0.05 3.7 f₂0.0 0.02 0.75 0.95 1.0 0.8 0.12 0.07 3.7

M1-M8 are eight active microphones and SUM is the summated signal afterband-pass filtering. The two frequencies are spaced comparativelyclosely. It is important that the amplitude of the summated signal foreach frequency is the same. This is an important property of band-passfilters, as this is a contributing factor for achieving thefrequency-independent directivity.

FIG. 6 shows the sensitivities of the individual microphones in amicrophone array according to the invention. The horizontal axis is thefrequency range under investigation. The vertical axis is the individualmicrophone number. The dark, or black, colour indicates the lowestsensitivity and the light, or white, colour indicates the highestsensitivity. A horizontal line through the diagram, for example the onedenoted BP11, corresponds to the band-pass filter for microphone M11,and in the same way, horizontal line BP5 corresponds to the band-passfilter for microphone M5. Therefore, the sensitivities of themicrophones depend on their positions and the frequency range they areintended for. Of course, there is a great number of possible band-passfilters for the individual microphones, and it goes without saying thatthe resulting directivity of the microphone array depends on theselection. If all band-pass filters of FIG. 4 are altered so that theypossess a narrower bandwidth with the same centre frequency, the activeaperture, being a function of the frequency, becomes smaller, and areduced number of microphones are active for any given frequency. As aresult, directivity decreases. If, on the other hand, the bandwidth isincreased, the active apertures become wider, and directivity isimproved.

In a system comprising all parts according to the invention, i.e, boththe frequency-dependent microphone positioning and the band-pass filtersfor the individual microphones, several interesting results areobtained. A first example is shown in FIG. 7. The illustrated examplecomprises a 60 cm microphone array having 40 microphones disposed withexponential spacing and intended for the frequency range 750 Hz to 44kHz, corresponding to about 6 octaves. In this case, no band-passfilters for the individual microphones of the microphone array are used.As is apparent, sensitivity to all frequencies is high along the centreaxis of the microphone array. At high frequencies, the sensitivity ofthe microphone array decreases considerably with the angle of incidencefor the sound, and thus, the microphone array achieves high directivityfor high frequencies. At the low frequencies, however, there is nosubstantial difference between the sensitivity of the microphone arraywith respect to sound incident along the centre axis of the microphonearray and sound incident at an angle to said centre axis. Thus, themicrophone array has poor directivity for those very low frequencies.

Regarding FIG. 8, a completely different result is shown. In FIG. 8, amicrophone array corresponding to the one of FIG. 7 is employed, but inthis case, band-pass filters are used for the individual microphones,said filters being adapted according to the frequency bands they areused for, as described above. The sensitivity of the microphone array tohigh frequencies is more or less identical to the sensitivityillustrated in FIG. 7, while the sensitivity of the microphone array tolow frequencies is substantially different. As is apparent, thesensitivity of the microphone array to sound incident at an angle to thecentre axis is substantially lower than the sensitivity of themicrophone array around the centre axis, even to low frequencies. Itshould be noted that the effect of the band-pass filters used is alsoapparent in the form of an attenuation at the highest and lowestfrequencies. In the end, the directivity of the microphone arrayaccording to the invention is based on a design where possible band-passfilters, physical size and desired directivity are all being taken intoconsideration. Thus, it is achieved that the microphone array has a highdirectivity in a large frequency range, said directivity at the sametime being substantially constant across a large frequency range. Itshould be noted that the sidebands are visible in both FIG. 7 and FIG.8, but that in FIG. 8 said sidebands are substantially attenuated.

It should be noted, of course, that the passbands and the cut-offfrequencies of the individual band-pass filters may be alteredregularly, thus apart from said microphone array having a highdirectivity in a large frequency range also allowing for the directivityof a microphone array to be altered regularly, so that the samemicrophone array can display different directivity characteristics,depending on the setting of said individual band-pass filters. It shouldalso be noted that the individual signals from the individualmicrophones of the microphone array may be reused so that the samemicrophone array may display a high directivity and a very smalldirectivity, depending on the processing of the signals, by using two ormore sets of band-pass filters simultaneously. The signals from theindividual microphones of the microphone array may also be recordedseparately and band-pass filtered at a later time, thereby determiningthe desired directivity at a later stage.

The invention is explained above on the basis of an elongated microphonearray. However, this is not the only embodiment that may be used. One ormore microphone arrays according to the invention may, for example, bearranged mutually perpendicular or with another mutual angle, therebyallowing a more detailed directivity sensitivity.

As illustrated in FIG. 10, a microphone arrangement 7 may consist of twoor more elongated microphone arrays 1. In this embodiment, themicrophone arrays 1 are disposed along radii of an imagined circle,where the reference ends 2 of the microphone arrays face centre C of theimagined circle. Thus, the individual microphones 5 are arranged inconcentric circles 8, the radial distance between the individualcircular arcs 8 corresponding to the spacing of individual microphones 5of elongated microphone arrays 1.

The spacing between microphones 5 on the circular arc on the innermostcircle has to substantially correspond to or be smaller than the radialdistance between the two circles closest to centre C. This means, thatthe greater the distance from the reference end 2 of the microphonearrays 1 to the centre C, the more microphone arrays 1 have to be usedin the microphone arrangement 7. Therefore, it is important to keep thelatter distance as small as possible or, in other words, to keep thecentre opening as small as possible. Preferably, the angles between theindividual microphone arrays 1 or radii are identical.

The signals from microphones 5 of the microphone apparatus 7 are allassociated with time delays selected in such a way that the effect ofthe microphone apparatus 7 is focused in at least one direction and/oragainst one punctiform area in front of the microphone apparatus.Band-pass filtering of the signals takes place with the summated signalsfrom the microphones 5 of the same circle 8 after having time delayedthe signals from the microphones 5. The time delays and band-passfilters may be selected in such a way as to enable simultaneous focusingin several directions with the same directivity efficiency. The same maybe accomplished by running the signals from individual microphones 5through several sets of time delays and/or several sets of band-passfilters.

The elongated microphone arrays 1 of the microphone arrangement do notnecessarily need to be identical. For example, only every secondmicrophone array may be identical, as illustrated in FIG. 11. Microphonearrays 1 may be assembled in such a way that the individual microphones5 of the microphone arrays 1 are only on every second concentric circle8. This way, a number of microphones may be dispensed with out losingthe directivity and/or focusing in question. Naturally, othercombinations are also possible, such as only every third or fourthmicrophone array 1 being identical.

As shown in FIG. 9, the time delays are used to apparently rotate theflat microphone array 6 and focusing it on a point in space outside themicrophone array 6. This apparent rotation is achieved by consideringthe actual position of the microphones and the positioning necessary toachieve the desired focusing and rotation. The time delays aredetermined at by means of the apparent distances the microphones have tobe moved in order to achieve the rotation and focusing. The alteredmicrophone apparatus 7 can focus on a punctiform sound emitter whileachieving the same advantages as with the elongated microphone arrayaccording to the invention. The same signals from individual microphonesmay be used more than once, and therefore it is possible to focus onseveral points at the same time. It is also possible to use differentband-pass filters and to obtain different directivities for theindividual focal points.

Above, the invention has been described by way of several exemplaryembodiments. However, it is possible to make alterations to theillustrated examples without deviating from the scope of the invention.For example, it is conceivable to dispose the individual microphones ona paraboloid or cone in a microphone arrangement, which may possiblyprovide several new effects, such as an attenuation of the rear sidesensitivity of the microphone arrangement. It is also conceivable toposition a single microphone in the centre of the microphone apparatus.

1. Microphone array (1) for achieving a substantiallyfrequency-independent directivity using a plurality of microphones (5)disposed along a rectilinear array, where: the rectilinear array is atleast as long as the wavelength of the lowest frequency, where a usefuldirectivity is desired, the rectilinear array has a first end (2) and asecond end, the microphones close to the first end (2) are intended forthe highest frequencies and the microphones close to the second end areintended for the lowest frequencies, the position of the microphones isgiven by the formula: $l_{n} = {l \cdot 2^{- {(\frac{N - n}{d})}}}$wherein l is longest wavelength, for which frequency-independence isdesired, N is the maximum number of microphones, l_(n) is the positionof the n′th microphone with respect to the end of the microphone array,which is intended for the highest frequencies, and d is the number ofmicrophones per octave, the signals from the individual microphones (5)are time-delayed so that phase differences or propagation timedifferences caused by the spatial position of the microphones (5) aretaken into account, characterised in that the signals from theindividual microphones (5) each independently are band-pass filtered,the band-pass filters for the individual microphones being digital witha pole position, resulting in a constant group propagation time withinthe entire frequency range used and ensuring that the signals from theband-pass filters are in phase, wherein the ratio between the bandwidthsand centre frequencies of the individual band-pass filters is constant,and wherein the signals after band-pass filtering are summated forobtaining the output signal.
 2. Microphone array according to claim 1,characterised in that signals from the individual microphones of themicrophone array are recorded prior to being time-delayed and band-passfiltered, and that these signals are time-delayed and band-pass filteredat a later stage for obtaining the desired directivity.
 3. Microphonearrangement, comprising at least two microphone arrays (1) according toclaim 1, characterised in that the at least two microphone arrays (1)are arranged in one plane.
 4. Microphone arrangement according to claim3, characterised in that the microphone arrays (1) are disposedsubstantially along radii extending from the centre C of an imaginedcircle, the first ends (2) facing the centre C.
 5. Microphonearrangement according to claim 4, characterised in that at least twodifferent individual microphones (5) from different microphone arrays(1) are disposed along imagined concentric circles (8) having the samecentre C.
 6. Microphone arrangement according to claim 4, characterisedin that the shortest circular arc spacing between microphones (5) on thecircle closest to the centre C substantially corresponds to or issmaller than the radial distance between the two circles closest to thecentre C.
 7. Microphone arrangement according to claim 3, characterisedin that the signals from the individual microphones (5) are eachassociated with time delays selected in such a way that the effect ofthe microphone arrangement is focused in at least one direction and/oragainst one punctiform area in front of the microphone apparatus. 8.Microphone arrangement according to claim 7, characterised in that theindividual band-pass filterings are carried out for summated signalsfrom the individual microphones (5) on the same circular arc (8) afterthe signals from the microphones (5) have been time-delayed. 9.Microphone arrangement according to claim 8, characterised in that thesignals from individual microphones (5) are run through several sets oftime delays and/or several sets of band-pass filters.